1. Field of the Invention
This invention relates to an isothermal forging die formed of ceramic and more particularly to a ceramic isothermal die suitable for being heated to and maintained at the forging temperature of a heat resistant metal such as a titanium alloy or a nickel-based alloy during the period that the metal is being forged.
2. Prior Art Statement
Most metallic materials having great high-temperature strength such as titanium and nickel-based alloys are characteristically difficult to process. One effective way of processing such materials is by the isothermal forging method in which the forging die is heated to the forging temperature of the metal concerned (to 900.degree. C. in the case of a titanium alloy and to 1050.degree. C. in the case of a nickel-based alloy).
Conventionally such isothermal forging has been carried out using a die made of a molybdenum alloy exhibiting a high coefficient of thermal conductivity. In the atmosphere, the molybdenum alloy oxidizes at temperatures over 300.degree. C. and at temperatures exceeding 600.degree. C. the oxides vaporize, much in the manner of sublimation. Because of this, the forging has been carried out in a protective atmosphere or a vacuum, making it necessary to use a huge chamber capable of accommodating the forging equipment and the like.
The inventors therefore considered ceramic material as a possible replacement for the molybdenum alloy of the die. Use of a ceramic die would enable the forging operation to be conducted in the atmosphere and, as a result, would make isothermal forging more generally usable. To this end, the inventors conducted experiments and pursued various avenues of research in connection with ceramic materials. However, they found that practical application was beset with numerous problems relating to thermal stress and the like.
Most isothermal dies are disk-shaped and are generally constituted of two disk-shaped die halves whose mating surfaces have been formed with die cavities At the time of isothermal forging, the two die halves are mated with the workpiece to be forged sandwiched between them and the result is loaded into a forging press and heated. The heating thus proceeds from the periphery not formed with a die cavity. Where the die is formed of ceramic, the generally low coefficient of thermal conductivity of this material tends to cause a steep temperature gradient at the time of heating and cooling. As a result, when the die is heated or cooled from its periphery, a large temperature gradient occurs in the radial direction and this produces a large thermal stress in the ceramic material. When this stress becomes greater than what the ceramic can stand, the ceramic breaks, making the die unserviceable. This phenomenon is dominated by the temperature difference in the die, particularly that in the radial direction, and tends to occur mainly at the start of heating and cooling.
For using ceramic as a material for a forging die, therefore, it is necessary to provide the die with a structure which enables it to avoid excessively large stress and, simultaneously, to bear up under the forging load.
In view of the foregoing, the object of the present invention is to provide a ceramic isothermal forging die which suppresses the occurrence of excessive stress due to large temperature gradient and can be used for isothermal forging not only in a vacuum or protective atmosphere but also in the atmosphere.